1. Field of the Invention
The Invention relates to a lifting column, preferably for height adjustable tables, comprising a guide and a drive unit, where the guide comprises at least a first member and a second member, where the members are mutually telescopically arranged relative to each other, and where a first set of sliders is arranged on an internal side of the first member at the end of the first member into which the second member extends and may be displaced out of, and where a second and third set of sliders are arranged on the outside of the second member at the end of the second member which extends into the first member. The invention further relates to a height adjustable table equipped with at least one such lifting column.
2. The Prior Art
For explanation of the invention reference is made to height adjustable or so-called sit/stand work tables, where the underframe comprises an electrically driven lifting column at each side of the work table and where the lower end of the lifting columns are furnished with a foot. A table top is secured to an upper end of the lifting columns either directly or by means of a top frame. The lifting columns comprise a guide which typically consists of two or three mutually telescopically arranged members. In an embodiment, which is probably the most commonly used so far, the member having the largest cross section is placed at the bottom. In another embodiment the lifting column is “turned around”, such that the member having the smallest cross section is at the bottom; this embodiment is also known as “up-side down”. In the first mentioned embodiment, the one with the member having the largest cross section at the bottom, the upper end of the lifting column may be equipped with a cross member, connecting the two lifting columns, which gives a good sideways stability of the table. In some embodiments without a cross member the upper end of the lifting columns is furnished with a strut between the lifting column and the table top or in connection with a motor housing on the column, located perpendicular to the guide.
The stability of the table is a crucial factor. A particular problem is vibrations of the table when it is exposed to a horizontal force. The horizontal force can be a result of the work being performed at the table or machines, e.g., a printer, located thereon. The back and forth movements of the printer head can cause vibrations of the table. DIN 4554, relating to office furniture, determines a threshold value for the allowable vibrations, just as a test for testing the vibration properties of the table, popular known as “the eraser test” is prescribed. The vibrations should not be too big and the table should settle again quickly.
Between the individual members in the guides of the lifting columns sliders are arranged. These sliders serve the purpose of compensating for manufacturing tolerances of the members. For that purpose the sliders may be designed with an adjustable thickness, such that they can be adjusted to the present groove between the members, cf., e.g., EP 1 250 866 A1 to Assenburg BV, EP 1 004 784 A1 to Magnetic Elektromotoren AG and DE 298 12 762 U1 to Phoenix Mecano Komponenten AG. Alternatively, a range of sliders with various thicknesses fitting the present groove between the members may be provided, cf., e.g., WO 03/047389 A1 to Linak A/S, where it is further suggested to make a local deformation opposite the slider for further equalization of the tolerances. The sliders also serve the purpose of keeping the friction between the individual members as low as possible. The amount of the friction has a direct influence on the force and energy needed to displace the guide. Popularly speaking, the more friction the larger motor and thus energy consumption is required.
Another factor for the stability of a lifting column is the overlapping of the individual members in the guide, i.e., the portion of the member protruding into the previous member. A larger overlapping of the telescopic members results in a better stability of the lifting column. There is, however, a limit to how large the overlapping can be. The adjustment of the height of the tables is also determined through various standards. According to the European standard EN 527, the work table must be adjustable from 60 cm to 120 cm in height, but some standards prescribe an even larger height, e.g., the Dutch standard NEN 2449 which prescribes an interval from 62 cm to 128 cm. This naturally limits the size of the overlapping between the members. In order to meet the requirement for the minimum and the maximum height, it is necessary to have three-part columns, i.e., columns having a guide consisting of three telescopic members. Due to tolerances the innermost member can yield a little relative to the intermediate member, which in turn can yield a little relative to the outermost member.
Especially for work tables in the nature of desks where the design is of vital importance, it is a pronounced wish that lifting columns appearing as table legs are slim. The ratio between a relatively small cross section relative to the length results in the lifting columns themselves appearing with a certain elasticity.
As regards work tables the problem of vibrations is particularly significant in the longitudinal direction of the table, while the vibrations in the transverse direction of the table are usually not a problem. Part of the explanation is that the feet extend in the transverse direction of the table while the dimension of the feet in the sideways direction in comparison is relatively small. As to lifting columns with guides having a rectangular cross section, the lifting columns are usually also positioned such that the broadside faces across the table while the narrow side faces in the longitudinal direction of the table.
The purpose of the invention is to provide a solution for reducing vibrations of tables as a result of horizontal forces.
This is achieved according to the invention with a lifting column where the second set of sliders, which describes a first plane, has been applied a given first prestressing between the first member and the second member, while the third set of sliders, which altogether describes a second plane which is different from the first plane, has been applied a given second prestressing between the first member and the second member, where the second applied prestressing is smaller than the first applied prestressing. Thus, it has turned out that this deliberate differentiation of the prestressing surprisingly enables a significant reduction of the vibrations in the lifting columns.
The prestressing of the sliders can be done in various ways, e.g., by pressing sliders with a certain thickness into the groove between the members, where experiments have determined how thick the sliders should be in order to obtain the desired prestressing. To achieve the desired, prestressing places great demands on the manufacturing tolerances. Another possibility is to use adjustment screws for the prestressing, this is however not particularly manufacturing-friendly. In a preferred embodiment of the invention the prestressing is created by an embossing of the member opposite the sliders. By creating a deformation of the side wall of the member, the desired prestressing may quite accurately be achieved. The embossing can be made both on the outer side of the outermost member as well as on the inner side of the innermost member. With an embossing on the Inner side visible marks on the outer side of the guide appearing as table legs are avoided.
In an embodiment two sets of sliders are placed after each other at the end of the member extending into the second member and the sliders in the first plane and in the second plane are offset. As it turns out, this has proven to provide a good dampening of the vibrations. In the plane where the small prestressing is provided, it has turned out that this dampening is further improved with a slightly larger prestressing of the bottommost sliders than the topmost sliders of the two sets of sliders.
As to lifting columns having three or more members, the invention has proven that a prestressing of the sliders between a fixed member and the first successive member arranged telescopically relative to the fixed member is sufficient. The prestressing between the additional subsequent members has turned out not to be of much significance. Since the prestressing is only provided at the fixed member, the manufacturing costs and the cost of tools are reduced.
Through experiments an expedient embodiment of the invention has been found, where the large prestressing is of a size resulting in a friction of between 15-30 kg between the respective sliders and the member sliding against these, while the small prestressing is of a size resulting in a friction of 0-5 kg between the respective sliders and the member sliding against these.
In an embodiment the telescopic members in the lifting columns are constituted by pipe profiles which may have a random cross section, but a square cross section provides well-defined planes for the prestressing of the sliders and the effect of the invention thus becomes unambiguous. In an embodiment the members have a rectangular cross section, where one side, the broad side, in the cross section is significantly larger than the width of the other side, the narrow side. A rectangular cross section allows for an even more unambiguous differentiation of the small and the large prestressing relative to each other. In an embodiment the broadside of the profiles in the guides has a width, which is about one and a half times larger than the narrow side or even larger.
When the lifting columns are mounted in a structure they are expediently placed such that the second, the small, prestressing faces in the direction in which the transversal forces which should be dampened occur.
The first, the large, prestressing in the lifting column(s) is provided in a direction across the table top and the second, the small, prestressing is provided along the table top.
The invention further relates to a height-adjustable work table comprising a table top carried by at least one lifting column and where the table top has two long sides, where one long side constitutes a rear side and the other a front side, and where the user of the table is placed at the front side, just as the table top has two ends extending between the front side and the rear side, and where the first, the large, prestressing in the lifting column(s) (5) is provided in a direction across the table top and that the second, the small, prestressing is provided in the longitudinal direction of the table top.